This invention relates to a magneto casing of a sealed construction for an engine in which the internal pressure undergoes changes as if the magneto casing breathed as the engine operates.
In engines such as marine engines wherein large quantities of water tend to flow into the engine rooms, magneto casings are constructed in a manner to have a sealed construction for preventing the invasion of water so as to provide protection to the magneto against water. The magneto casings of the sealed construction of the prior art have encountered problems which are presently to be described.
Generally, the magneto is mounted on one end portion of the crankshaft which projects outwardly of the crankcase. Thus an oil seal is provided between the crankcase and the magneto casing for preventing leakage of oil from the crankcase side to the magneto casing side. The oil seal is effective to prevent leakage of oil but has no effect in cutting off the passage of air. Consequently, the internal pressure of the magneto casing undergoes changes as if the magneto casing breathed due to variations in the pressure in the crankcase and variations in the temperature in the magneto casing as the engine operates. Such changes in the internal pressure of the magneto casing are especially marked in the case of a two-cycle engine in which the crank chamber is sometimes subjected to a negative pressure of over 0.3 kg/cm.sup.2 when compression and expansion strokes are repeated. This causes invasion of the magneto casing having a starter by water.